A man wakes from a nap, walks up to the woman he has been married to for fifty years, and asks who she is. He can describe her face accurately. He knows what his wife looks like. He is simply unshakably certain that this particular woman is not her.
That is Capgras syndrome, and for a century it has been filed mostly under psychiatry. A retrospective cohort study from the Mayo Clinic, published in the journal Brain, is the greatest effort yet to determine what actually drives it. The answer is not the one the textbooks emphasize.
